1. Field of the Invention
This invention relates to imaging materials, particularly imaging materials which use negative acting photopolymerizable compositions, and more particularly to subbing layers useful on bases which are associated with photopolymerizable compositions.
2. Background of the Art
Photopolymerizable compositions can be used in a number of different types of imaging processes. One important commercial use of photopolymerizable compositions is in the technical area of color proofing, particularly in overlay color proofing. This type of process involves the formation of individual color sheets (e.g., cyan, magenta, yellow and black) from their respective color separation negatives and then overlaying the sheets in register to give a proof of the intended printed image. This type of procedure is disclosed for example in U.S. Pat. Nos. 3,136,637 and 3,671,236. The individual color layers can be formed by pigmented photopolymerizable compositions which, after exposure, are washed in developer solutions with mild scrubbing to provide the individual color images.
In addition to the overlay process, transfer processes can be used in the formation of proofing images. In this process, the individual color images are formed on carrier sheets and transferred, one at a time, onto a surface of a receptor sheet. The individual color images are transferred in register to form a prepress proof of the intended printed image. This type of process is shown in U.S. Pat. No. 4,482,625.
One particular area of problems that has been encountered in photopolymerizable (and photosolubilizable) compositions for use in prepress color proofing has been the critical balance in properties necessary between the substrate and the photopolymerizable composition. The composition must adhere reasonably well to the substrate before imaging, yet be removable in unexposed areas upon development. The exposed areas must also adhere well to the substrate and must adhere more strongly than the unexposed areas. The polymerized areas must also be capable of thermoplastic bonding or adhesive bonding to a receptor sheet with a bond strength greater than its bond strength to the carrier substrate. Without these balances in properties, there would be no faithful reproduction of images and the products would be readily subject to mechanical damage in even the mildly vigorous development processing which color proofing sheets undergo. A major improvement in this area was made by the introduction of polyamide subbing layers between the substrate and the photosensitive layer as shown in U.S. Pat. No. 3,778,272. There the use of methylolated or etherified polyamides soluble in a mixture of alcohol and water is described.
In order to provide higher photographic speed to photopolymeric compositions, it is generally necessary to use higher proportions of monomers and binders with large numbers of photopolymerizable groups thereon. Such binders and compositions using them are shown, for example, in U.S. Pat. Nos. 4,304,923 and 4,228,232. The compositions of these patents having higher monomeric and oligomeric components with lower proportions of film-forming binders provide the higher speed compositions. However, higher concentrations of monomers creates another problem, migration of components from the photosensitive layer into the substrate and/or subbing layer. The migration of the monomers usually carries dyes, photoinitiators and other additives with it. This can greatly vary the adherence properties of the photosensitive layer to the substrate, both before and after exposure. The speed of the photosensitive layer can also adversely vary with time because of the change in the composition of the photosensitive layer due to the uneven migration of components. Even the use of polyamide subbing layers on these types of higher speed photopolymerizable compositions (as shown in U.S. Pat. No. 4,482,625, Example 4) does not solve this migration problem.